1. Field of the Invention
This invention relates to ZSM-48 having a new crystal morphology and a method for its preparation.
2. Description of the Prior Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Zeolites typically have uniform pore diameters of about 3 angstroms to about 10 angstroms. The chemical composition of zeolites can vary widely and they typically consist of SiO.sub.2 in which some of the silica atoms may be replace by tetravalent atoms such as Ti or Ge, by trivalent ions such as Al, B, Ga, Fe, or by bivalent ions such as Be, or by a combination of any of the aforementioned ions. When there is substitution by bivalent or trivalent ions, cations such as Na, K, Ca, NH.sub.4 or H are also present.
Zeolites include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example, an alkali metal, an alkaline earth metal cation, or an organic species such as a quaternary ammonium cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li is equal to unity. One type of cation may be exchanged either entirely or partially by another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are usually occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic aluminosilicates. These aluminosilicates have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite ZK-4 (U.S. Pat. No. 3,314,752), zeolite ZSM-5 (U.S. Pat. No. 3,702,886), zeolite ZSM-11 (U.S. Pat. No. 3,709,979), and zeolite ZSM-12 (U.S. Pat. No. 3,832,449), merely to name a few.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with SiO.sub.2 /Al.sub.2 O.sub.3 ratio of from about 2 to about 3; zeolite Y, from about 3 to about 6. In some zeolites, the upper limit of SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one such example wherein SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least five. U.S. Pat. No.3,941,871 discloses a crystalline metal organosilicate essentially free of aluminum and exhibiting an X-ray diffraction pattern characteristic of ZSM-5 type aluminosilicates. U.S. Pat. Nos. 4,061,724, 4,073,865 and 4,104,294 describe microporous, crystalline silicas or organosilicates of varying alumina and metal content.
U.S. Pat. No. 4,423,021 to Rollmann et al. describes a method for synthesizing silico-crystal ZSM-48 using a diamine having four to twelve carbons as the directing agent. The composition is described as a silico-crystal and it includes very little, if any aluminum.
U.S. Pat. Nos. 4,397,827 and 4,448,675 to Chu also describes method for synthesizing a silico-crystal ZSM-48 including very little, if any, aluminum. The synthesis utilizes a mixture of an amine having from two to twelve carbons and tetramethylammonium compound as the directing agent.
U.S. Pat. No. 5,075,269 to Degnan et al. describes silico-crystal ZSM-48 prepared with organic linear diquaternary ammonium compound as the directing agent. The crystal morphology is illustrated in FIGS. 3 and 4 of the patent and is described as having platelet-like crystal morphology at high silica/alumina ratios and aggregates of small irregularly shaped crystals at silica/alumina ratios below 200. In U.S. Pat. No. 5,075,269 this is compared with the crystal morphology of Rollmann et al. (U.S. Pat. No. 4,423,021) in FIG. 1 and Chu (U.S. Pat. No. 4,397,827) in FIG. 2. FIGS. 1 and 2 show a rod-like or needle-like crystal morphology which is random and dispersed.
ZSM-48 is also described by R. Szostak, Handbook of Molecular Sieves, Van Nostrand Rheinhold, New York 1992, at pp. 551-553. Organics are listed as diquat-6, bis (N-methylpyridyl)ethylinium, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, 1, 4, 8, 11-tetra-aza-undecane, 1, 5, 9, 13-tetra-aza-undecane, 1, 5, 8, 12-tetra-aza-undecane, 1, 3-diaminopropane, n-propylamine/TMA.sup.+, hexane-diamine and triethylamine. There is no suggestion of ethylenediamine. In addition, the crystal habit is described therein as bundles of needles and there is no suggestion of radial morphology.
Accordingly, the prior art utilizes relatively complex and expensive organics. It is an object of the present invention to overcome disadvantages of the prior art using a less expensive and readily available directing agent to make ZSM-48. This reduces manufacturing costs and provides a superior ZSM-48 product.